This invention relates to a system for limitation of the output current from a speed controller for three-phase asynchronous electric motors comprising a current converter bridge in which the electronic switches are controlled by a microcontroller.
A speed controller for a three-phase asynchronous motor comprises a voltage converter together with an uncontrolled rectifier and a capacitive filter. A typical converter is controlled by Pulse Width Modulation that consists of outputting a series of positive or negative fixed amplitude pulses modulated in width to the motor. There are several known processes for achieving Pulse Width Modulation. In sine-delta modulation, a sinusoidal reference wave with the required voltage frequency and a proportional amplitude is compared with a delta modulated wave with a significantly higher frequency. The intersections of the two curves give the closing and opening times of electronic switches in the converter respectively and in real time. Hysteresis modulation is another means of achieving Pulse Width Modulation.
U.S. Pat. No. 4,904,919 describes a system combining the advantages of the PWM sine-delta converter and the advantages of the PWM-hysteresis. A circuit measures the current and compares it with a limiting positive value and a limiting negative value of the current. If the current exceeds its limiting positive value, a predetermined negative value is added to the current reference. If the current drops below this limit, the action of the said negative value is still valid. In the fall phase, there is a threshold below which the action of the negative value is deactivated.
In this dual system, hysteresis modulation operates in series with sine-delta modulation. If the current drops below its limit, hysteresis modulation is inactive and sine-delta modulation is active. However if the current exceeds its limit, hysteresis modulation is active in the sense that it modifies the reference current that then acts on the regulation loop, and therefore on the PWM reference voltage. In other words, in practice switching to hysteresis mode activates it, and switching to the other direction deactivates it while leaving sine-delta PWM active.
This solution clips the current in each phase separately which induces a vibration motor torque. The current oscillates within a wide band which generates additional oscillations in the torque and thus significantly reduces the efficiency of the motor.
In terms of converter components, if each phase is limited independently, the result is that each power component (switches, diodes) in the converter can be protected individually. But the total dissipated power depends directly on the modulus of the current vector. The modulus of this current vector always remains greater than the limiting value and the load on power components is thus greater such that their life could be affected.
The purpose of this invention is to provide a function for instantaneous limitation of the current at the output from the controller while keeping the currents sinusoidal. This function is made by software and the micro-controller independently of the type of the main control used in the speed controller. It protects the converter bridge, eliminates torque ripples and thus extends the life of the motor. No additional hardware components are necessary, the layout of the current limitation only requires a small amount of memory space and little microcontroller execution time. Current limitation is more reliable, more intelligent and less expensive. During limitation, the modulus of the current vector is kept equal to the limiting value, which extends the life of power components in the converter.
The system according to the invention is characterized by the fact that the microcontroller circuit comprises means of calculating the modulus of the current vector from measurements of motor phase currents, and comparing it with a limiting set value in order to obtain a limitation error and means of calculating a correction voltage that is added to the control voltage applied to the motor.
According to one characteristic, the system comprises means of starting from the limitation error and using a floating action controller to calculate a control variable, and means of multiplying this control variable by phase currents in order to obtain the correction voltage for each phase to obtain sinusoidal limitation currents.